b-Lactam susceptibility of anaerobic isolates from infants' nasopharynges during acute otitis media episodes

Objectives: Various anaerobic species, many of which produce b-lactamases, are typical inhabitants of the mouth but can be frequently isolated from infants' nasopharynges during acute otitis media (AOM). When b-lactamase producers are involved in infections, as etiologic agents or even innocent bystanders, they may cause a treatment failure by inactivating b-lactams used for antimicrobial therapy. The aim of this study was to determine in vitro susceptibilities to four b-lactam agents of anaerobic isolates collected from the nasopharynx during AOM episodes. Methods: Altogether 92 microaerophilic/anaerobic isolates were available from 32 nasopharyngeal aspirates collected from 22 infants during AOM episodes by 2 years of age (FinOM Cohort Study). b-Lactamase production was assessed by using a chromogenic cephalosporin disk test. Minimal inhibitory concentrations (MICs) of the isolates, including potential b-lactamase producers and non-producers, were tested to three b-lactams, penicillin G (Pen), cefaclor (Cef), and cephalotin (Ceph), and one b-lactam/b-lactamase inhibitor combination, amoxicillin/clavulanate (Amo/cla), by the NCCLS approved agar dilution method. Results: 25/35 Prevotella, 8/12 Porphyromonas, and 1/10 Fusobacterium isolates produced b-lactamase. The MICs to Pen of b-lactamase-producing isolates ranged between 1-64 mg/ml for P. melaninogenica group, 16-32 mg/ml for nonpigmented Prevotella spp., 1-8 mg/ml for P. catoniae, 8-16 mg/ml for P. levii-like organisms, and 32 mg/ml for F. nucleatum. Generally, the MICs to cephalosporins Cef and Ceph were higher than to Pen. All isolates were susceptible to Amo/cla but the MICs were relatively higher among b-lactamase-producing isolates than non-producing isolates. No b-lactamase production was observed among 6 Campylobacter, 3 Capnocytophaga, 6 Veillonella, 7 Actinomyces, 4 Clostridium, 5 Micromonas, and 4 Peptostreptococcus isolates tested. All Veillonella isolates showed decreased susceptibility to b-lactams, MICs to Pen varying between 1-8 mg/ml. Conclusions: Decreased susceptibility to b-lactam antibiotics due to b-lactamase-mediated and non-mediated resistance mechanism is a common phenomenon among anaerobes present in infants' nasopharynges during AOM.